Add missing return types

Move verifyModifier/verifyClassModifier to Modifiers class

Now that the Modifiers are in a separate class, these *internal*
verification methods should also be moved there.

Require strict_types in php-cs-fixer config

Introduce separate Modifiers class

Use PhpParser\Modifiers::PUBLIC instead of
PhpParser\Node\Stmt\Class_::MODIFIER_PUBLIC, etc. Old constants
of course remain available.

Fix

Introduce separate Modifiers class

Use PhpParser\Modifiers::PUBLIC instead of
PhpParser\Node\Stmt\Class_::MODIFIER_PUBLIC, etc. Old constants
of course remain available.

Fixes #476.

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